I know of two on my server. One was a 5 shaman team in a guild called Multibox (or something similar) who used to hang around outside the arena master a lot, I'm not a pvper myself so figured I wouldn't have much to talk to them about. Second one was recently when I was questing on my 2 shaman, noticed my health drop out of combat and saw a priest had buffed me and targetted them to thank them (if I ever get a gamepad I'm binding emotes to a few keys so both toons can be nice), then I noticed motw on me and saw a druid just behind the priest.

They mounted up at the same time and rode off with the druid in the right place to be following the priest (rather than two unique players), didn't see them fighting though so it may just have been two friends playing together. They didn't whisper anything to me (and I'd been fighting so they would have known I was a MB) and I think they were in one of the foriegn guilds on my server so i didn't want to whisper them in case they weren't comfortable speaking in English. Even if they weren't a boxer it's nice to have someone buff both of my toons, I just felt bad that I couldn't buff them back.

Personally I MB (not very often) a) because it's faster than levelling two characters individually and b) because it's a bit different to levelling one at a time and after levelling a few toons it's nice to have a change of pace.